1. Field of the Invention
The present invention relates to a reference current output device and a reference current output method. The present invention particularly relates to a reference current output device and reference current output method, that generate a reference current that does not change dependent on changes in temperature.
2. Description of the Related Art
Japanese Patent Application Laid-Open (JP-A) No. 2006-262348 discloses a proportional-to-absolute-temperature (“PTAT” hereinafter) current output device that generates and outputs a current that does not change dependent on changes in temperature. The PTAT current output device recited in JP-A No. 2006-262348 generates a current that does not change dependent on changes in temperature, by counteracting a current that has a positive temperature gradient from a band gap circuit with a current that has a negative temperature gradient, and using a p-channel metal oxide semiconductor (MOS) field effect transistor (hereinafter, referred to as PMOS transistor) to output the current as a reference current having a predetermined temperature gradient. In the present specification, the meaning of the term “temperature gradient” is intended to include a rate of change of current value with absolute temperature.
However, characteristics of reference currents obtained by using the PTAT current output device recited in JP-A No. 2006-262348 may vary greatly due to variations in the sizes of transistors mounted in the PTAT current output device. Accordingly, in the PTAT current output device, a use of the circuit configuration as illustrated in FIG. 10 to adjust a characteristic of a reference current, have been investigated. FIG. 10 illustrates an invested structural example of the current output device 100. As illustrated in FIG. 10, the current output device 100 includes a reference voltage generation circuit 102 and a reference current generation circuit 104.
The reference voltage generation circuit 102 includes pnp-type bipolar transistors (hereinafter, referred to simply as bipolar transistors) 106 and 108, PMOS transistors 110 and 112, a resistor 114, an operational amplifier 116 and an output terminal 118. A ratio of the size of the bipolar transistor 106 to the size of the bipolar transistor 108 (a transistor ratio) is set as (size of the bipolar transistor 106):(size of the bipolar transistor 108)=1:N (which is a value greater than 1).
In the reference voltage generation circuit 102 with this configuration, a reference voltage having a predetermined voltage, that does not change dependent on changes in temperature, is outputted from the output terminal 118 by counteracting a voltage having a positive temperature gradient applied to the resistor 114, with a voltage having a negative temperature gradient applied to the bipolar transistor 108.
The reference current generation circuit 104 includes, a portion of the reference voltage generation circuit 102, n (an integer that is at least two) PMOS transistors 120A1 to 120An connected in parallel and having different sizes, PMOS transistors 120B1 to 120Bn, and an output terminal 122. A current mirror circuit is configured by the portion of the reference voltage generation circuit 102 and the PMOS transistors 120A1 to 120An. Hereinafter, in the descriptions, when it is not necessary to distinguish between the PMOS transistors 120A1 to 120A1, simply “the PMOS transistors 120A” are referred to, and when it is not necessary to distinguish between the PMOS transistors 120B1 to 120Bn, simply “the PMOS transistors 120B” are referred to.
In the reference current generation circuit 104 with the above configuration, a reference current i1(=i2) having a positive temperature gradient from the reference voltage generation circuit 102 is fed out through the output terminal 122 as a reference current i3 by the reference current generation circuit 104. This reference current i3 may be fed out as a current in correspondence with the current mirror ratio, set by selectively using the PMOS transistors 120A1 to 120An. Namely, the reference current i3, which is the current outputted from the PTAT current output device 100, may be adjusted by switching of the PMOS transistors 120B1 to 120Bn. Here, FIG. 11 is a graph illustrating absolute temperature characteristics before and after adjustment of the reference current generated by the current output device 100, wherein the horizontal axis represents the absolute temperature. As can be seen from FIG. 11, the current outputted from the current output device 100 can be aligned at a target characteristic point by the reference current i3 being adjusted by the PMOS transistors 120A, as described above.
However, in the current output device 100, because the PMOS transistors 120A have temperature characteristics (because respective currents outputted from the PMOS transistors 120A1 to 120An have different temperature gradients), temperature characteristics of the reference current generation circuit 104 may change when the output current is adjusted. Consequently, when the amount of the reference current i3 changes, the temperature gradient of the reference current i3 outputted from the reference current generation circuit 104 changes, for example, as illustrated in FIG. 11, and therefore, it is difficult to adjust the reference current i3 to have a predetermined temperature gradient. Note that, although an example of a conventional current output device that outputs a current having a positive temperature gradient have been described above, the same applies to a current output device that outputs a current having a negative temperature gradient, and therefore is difficult to adjust the outputted reference current to be a current having a predetermined temperature gradient.